The present invention pertains generally to the control of a laser beam. More particularly, the present invention pertains to systems and methods for controlling a laser beam by using an active mirror in accordance with wavefront analysis techniques. The present invention is particularly, but not exclusively, useful as a system and method for controlling a laser beam in ophthalmic applications wherein mechanically introduced optical aberrations require compensation.
In preparation for a so-called xe2x80x9cflap and zapxe2x80x9d (Lasik) ophthalmic laser surgery procedure, it is common practice to first create a stromal flap. Typically, this is done by mechanically cutting into the stroma to create the flap. To date, it has been the practice to accept the resultant incision as being substantially flat. Actually, however, the interface surface that results when the flap is created is not flat. Instead, due to irregularities in the sharpness of the cutting blade, due to imperfect aplanation for stabilizing the cornea during cutting of the flap, and due to the resistance between the stromal tissue and the cutting blade as the flap is created, the resulting incision typically includes several surface irregularities. These irregularities, unfortunately, are significant. Indeed, it happens that these irregularities are generally in the form of asymmetric undulations that may vary in amplitude by as much as ten microns, and extend over a distance of as much as a millimeter. As is well known, undulations (surface irregularities) of this magnitude will induce noticeable aberrations in any wavefront that passes through the surface.
It is known that an excimer laser has a relatively large spot size (e.g. approximately one millimeter in diameter). It is also known that an excimer laser will superficially photoablate an exposed layer of stromal tissue to a substantially uniform depth within the spot size. Consequently, any undulations of one millimeter that are initially present on an exposed surface of stromal tissue will persist and will still be present after the procedure has been completed. This, however, is good. Although the surface undulations will introduce aberrations during the procedure, these same aberrations will be cancelled when the flap is lowered onto the exposed surface.
Heretofore, whenever closed-loop control of an excimer laser has been employed during ophthalmic surgery, the contribution of the surface irregularities (undulations) to the total aberrations of the eye has been generally disregarded. Consequently, it has been the practice in earlier closed-loop control systems to generate a controlling error signal by comparing the extent of actual tissue photoablation to the predetermined amount of desired photoablation. Using wavefront analysis, this has been done. by identifying the actual distorted wavefront that is created by the stromal tissue (including the undulation contribution), and then comparing the distorted wavefront with a desired wavefront to generate the error signal. The desired wavefront, however, is normally determined off-site and is the result of a diagnostic examination. Thus, it is predetermined, and does not account for aberrations that are subsequently introduced by surface undulations (irregularities) when the flap is subsequently created at the time the procedure is to be performed. The result here is that the undulations on the exposed surface are removed along with the desired tissue removal.
In light of the above, it is an object of the present invention to provide a system and method for superficial photoablation of stromal tissue which compensates for the induced aberrations that are introduced when a flap of the cornea is mechanically created. It is another object of the present invention to provide a system and method for superficial photoablation of stromal tissue which generates an error signal that allows for more precise closed-loop control of photoablation during a procedure. Yet another object of the present invention is to provide a system and method for superficial photoablation of stromal tissue which is relatively easy to use, is simple to implement and is comparatively cost effective.
In accordance with the present invention, a closed-loop control system is provided for use in a so-called LASIK xe2x80x9cflap and zapxe2x80x9d procedure. Specifically, this closed-loop control is accomplished using wavefront analysis, and it is provided to compensate for optical aberrations that can be mechanically induced during the procedure. In the context of the present invention, it is important to appreciate that the patient will be examined and diagnosed prior to the corrective surgical procedure. Thus, the naturally occurring optical aberrations of an eye, that are unwanted and therefore need to be corrected, will be known and can serve as a starting point.
Importantly, the naturally occurring optical aberrations of an eye can be characterized as a distorted wavefront. As a corrective procedure is conducted, however, tissue will be removed from the stroma and the distorted wavefront will change accordingly. Deviations between the dynamically changing, distorted wavefront and a predetermined, desired wavefront (e.g. a plane wavefront) will then indicate the extent of corrections that are still required by the procedure. As appreciated by the present invention, in addition to all of this, there is a need to compensate for mechanically introduced optical aberrations.
In detail, the closed-loop control system of the present invention includes a source for generating an incising laser beam. Preferably, the incising laser beam is an excimer laser and will have a focal spot that is approximately one millimeter in diameter. As intended for the present invention, and implied above, the incising laser beam is only used to photoablate tissue from the exposed stromal surface after the flap has been lifted.
In addition to the incising laser source, the system of the present invention also includes an additional laser source for generating a diagnostic laser beam. Further, there is a deformable mirror for directing this diagnostic laser beam through the exposed stromal surface toward a focal spot on the retina of the eye. Preferably, the deformable mirror that is used for the present invention is of a type disclosed in U.S. application Ser. No. 09/512,440 which issued to Bille et al. for an invention entitled xe2x80x9cMethod for Programming an Active Mirror to Mimic a Wavefrontxe2x80x9d and which is assigned to the same assignee as the present invention. Light in the diagnostic laser beam will then be reflected from the focal spot on the retina and directed back through the exposed stromal surface.
A detector is positioned to receive light of the diagnostic beam that is reflected from the retina, back through the exposed stromal surface. At this point, it is to be appreciated that light in the diagnostic beam that is reflected from the retina will include two identifiable components. One component is a contribution that is characteristic of the distorted wavefront that was determined earlier during patient diagnosis. The other component will be a contribution from the optical aberrations that are introduced during the cutting of the corneal flap. Importantly, both component contributions can be characterized by distinct wavefronts.
For purposes of the present invention, the component contribution that is introduced during the cutting of the corneal flap is hereinafter referred to as an induced wavefront. On the other hand, the distorted wavefront has been previously determined during diagnosis. Therefore, the induced wavefront, which will not change during the procedure, can be determined by removing the distorted wavefront from the actually detected wavefront. Thus, the detector identifies and models, or generates, the induced wavefront that has characteristics of the mechanically induced optical aberrations. All of this is done using actual real-time characteristics of the cornea.
The system of the present invention also incorporates a compensator. Specifically, the compensator uses the induced wavefront to alter the desired wavefront. This is done by incorporating the induced wavefront with the desired wavefront, to create a rectified wavefront. In most applications, the desired wavefront will be a simple plane wavefront.
At this point it is helpful to recall that the system of the present invention is concerned with four specifically different wavefronts. These are: 1) a distorted wavefront that is characteristic of the uncorrected eye; 2) a desired wavefront for the corrected eye (this is the overall objective of the procedure); 3) an induced wavefront that is characteristic of the optical aberrations introduced during creation of the flap; and 4) a rectified wavefront that is a summation of 2) and 3).
In accordance with well known techniques for establishing closed-loop control, it is necessary to create an error signal. For the present invention, this function is accomplished by a comparator that compares the rectified wavefront (i.e. the desired wavefront incorporating the induced wavefront) with the distorted wavefront. Stated differently, the error signal represents the amount of correction required in the cornea (desired wavefront), and it includes a factor that compensates for the optical aberrations that were mechanically introduced during creation of the flap (induced wavefront). In effect, during the procedure the induced wavefront is preserved.
For the system of the present invention, the error signal is actually used for two purposes. For one, the error signal is used to reconfigure the deformable mirror and to thereby maintain a focal spot on the retina. This is important in order for the system to continuously determine the distorted wavefront as it is being altered by the photoablation of stromal tissue. For another, the error signal is used to deactivate the incising laser when the error signal is a null to thereby indicate that the proper correction has been accomplished.
Once the procedure has been completed, the flap is lowered onto the exposed, and now altered, stromal surface. Because the induced wavefront was preserved during the procedure, the undulations on the flap will mate with the preserved undulations on the exposed stromal surface. This will effectively cancel the optical aberrations that would otherwise be present.